1. Field of the Invention
The invention relates to a method and arrangement for embedding a watermark in an image. The invention also relates to a method and arrangement for detecting a watermark in an image. The expression "image" is understood to mean still images as well as video.
Digital watermarking is a technique used to certify the ownership of digital image or video contents. By imperceivably hiding such ownership information into digital content, it becomes possible to prevent piracy and illegal use of this content. Typical applications of watermarking include copy-protection of digital video and broadcast monitoring.
2. Description of the Related Art
Most presently used techniques for watermarking of digital images boil down to adding a pseudo-noise watermark pattern to an image. This addition may take place in the pixel domain or the frequency domain. Effectively, it means that luminance and/or color components of the watermarked image are slightly different from the original. The detection of a watermark is done by correlating the suspect image with the watermark pattern to be detected. Such a prior art method is disclosed, inter alia, in International Patent Application WO-A-98/03014.
A watermarking method must fulfill a number of basic requirements. Adding the watermark pattern may not affect the visual appearance of the image. There should be many different watermark patterns, since every copyright holder should have at least one distinct watermark. Detection of the watermark must be possible without using the original image. The detection must be fast so as to be able to detect watermarks in video, or to scan the World Wide Web for watermarked images. It should not be easy for attackers to remove the watermark. The method must further be robust to transformations or operations that can be applied to images or video, such as, color transformations, geometric transformations, such as, scaling, translation or rotation, data compression, such as, JPEG/MPEG, noise, embedding of other watermarks, gray scaling, image smoothing, dedicated transformations, A/D and D/A conversions, PAL/NTSC conversions, etc.
The above requirements are conflicting, and there are no methods known that fully satisfy all requirements. Many methods can easily be hacked by attackers who have a number of watermarked images at their disposal. Since detection is based on correlation (which is a linear operation), an attacker needs only a good estimate of the watermark, and he can use this to subtract the guessed watermark from images in order to mislead detection. Simple geometric transformations such as rotations or scaling can only be dealt with at the expense of considerable computing times during detection. It is also difficult to ensure sufficient variation of watermarks. The number of bits that can be embedded using one correlation is limited.